1. Field of the Invention
The present invention relates to a bipolar junction transistor (hereinafter abbreviated as BJT) layout structure, and more particularly, to an emitter-central BJT layout structure.
2. Description of the Prior Art
As is well known to those of skill in the art, a BJT device is a three-terminal device that essentially including an emitter, a base and a collector. In normal operation, the emitter-base junction will be forward biased while the collector-base junction reversed biased by externally applied voltages, and the device is driven in a forward active mode. Furthermore, BJT device can be manufactured using complementary metal-oxide-semiconductor (CMOS) process, and therefore plays an important role in band-gap voltage reference circuits. Accordingly, BJT device often serves as switching device and is often used in high-voltage, high-frequency, and/or high-power applications.
A typical npn-BJT device includes an n-collector which is an n-well formed in a p-substrate, a p-base which is a p-well formed in the n-well, and an n-emitter which is an n-doped region formed in the p-well. And a typical pnp-BJT device includes electric characteristic complementary to the npn-BJT device. In order to enhance emitter injection efficiency, it has been developed to form the emitter enclosed by the base pick-up and the collector. In this approach, a relatively small base current (that is the input current, IB) controls a relatively large collector current (that is the output current, IC). And a ratio of the collector current IC over the base current ID is referred to current-gain β or beta-gain β:β≡IC/IB 
The current-gain β increases in proportion to the collector current IC and is inversely proportional to the base current IB. That is, the current-gain β is increased if the collector current IC is increased and/or if the base current IB is decreased. More important, it is found that BJT performance is improved with current-gain β increase. Furthermore, the current gain β varies as a function of the base area or the collector area. It is found that the current gain β is increased when the base area is reduced or when the collector area is increased. However, such increase is limited because the base area cannot be increased without violating a design rule.
In view of the foregoing, it is desirable to provide a BJT layout structure that can efficaciously increase the current gain β without violating the design rule.